text: The National Atmospheric Deposition Program, or NADP, was initiated by the State Agricultural Experiment Station in 1977 to monitor the effects of atmospheric deposition on crops, rangelands, forests, surface waters, and other natural and cultural resources. The initial goal was to provide regional data for the deposition of acids, nutrients, and base cations (including temporal trends/amounts and geographic distributions).
  In 1978, the first NADP sites began collecting weekly precipitation samples. In the early 1980s, the National Acid Precipitation Assessment Program (NAPAP) was established, and began to work in collaboration with NADP in order to sustain a long term, quality-assured precipitation monitoring network. This unification brought on a major expansion as well as newfound federal agency support. Today, the NADP National Trends Network (NTN) has more than 250 sites.
  In response to emerging issues, the NADP established an additional two networks in the 1990s: The Atmospheric Integrated Research Monitoring Network (AIRMoN), which collected daily samples at five sites, and the Mercury Deposition Network (MDN), which has more than 80 sites (six of which are located in Canada). The MDN collects wet deposition data for both total and methyl mercury in precipitation.
  In 2009, the Atmospheric Mercury Network (AMNet) was formed as a fourth network, and as a subset of some MDN sites. The network uses continuous automatic measurement systems to monitor gaseous and particulate concentrations of atmospheric mercury. The Ammonia Monitoring Network (AMoN) was added as a fifth network in October 2010, and it currently has more than 100 sites. AMoN monitors ammonia gas concentrations across the United States to provide consistent and lasting data. The Mercury Litterfall Network (MLN) was approved as the sixth network in 2021 with 22 sites. MLN provides estimates of mercury dry deposition in forested landscapes using passive collectors.